Area array modulation and lead reduction in interferometric modulators

ABSTRACT

A light modulator is arranged as an array of rows and columns of interferometric display elements. Each element is divided into sub-rows of sub-elements. Array connection lines transmit operating signals to the display elements, with one connection line corresponding to one row of display elements in the array. Sub-array connection lines electrically connect to each array connection line. Switches transmit the operating signals from each array connection line to the sub-rows to effect gray scale modulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/731,989, filed on Dec. 9, 2003 and entitled Area Array Modulation andLead Reduction In Interferometric Modulators, now U.S. Pat. No. ______.The entire disclosure of this application is hereby incorporated byreference.

BACKGROUND

Interferometric modulators, such as the iMoD™, modulate light bycontrolling the self-interference of light that strikes the frontsurface of the modulator. These types of modulators typically employ acavity having at least one movable or deflectable wall. This deflectablewall moves through planes parallel to the front wall of the cavity—thewall that is first encountered by light striking the front surface ofthe modulator. As the movable wall, typically comprised at least partlyof metal and highly reflective, moves towards the front surface of thecavity, self-interference of the light within the cavity occurs, and thevarying distance between the front and movable wall affects the color oflight that exits the cavity at the front surface. The front surface istypically the surface where the image seen by the viewer appears, asinterferometric modulators are usually direct-view devices.

Typically, interferometric modulators are constructed of membranesformed over supports, the supports defining individual mechanicalelements that correspond to the picture elements (pixels) of an image.In a monochrome display, such as a display that switches between blackand white, one element might correspond to one pixel. In a colordisplay, three elements may make up each pixel, one each for red, greenand blue. The individual elements are controlled separately to producethe desired pixel reflectivity.

In one example of operation, a voltage is applied to the movable wall ofthe cavity, causing it be to electrostatically attracted to the frontsurface which in turn affects the color of the pixel seen by the viewer.A difficulty exists in producing modulators with accurate and repeatablemechanical properties, so that specific applied analog voltages producespecific analog displacements of the movable wall that acts as a mirrorat the back of the interferometric cavity. To produce accurate andrepeatable color combinations, typical modulators use only binarydisplacement of the movable mirror. In this mode of operation any givenmovable mirror-wall will be found at rest in either its quiescent statewhere it produces one of the color states mentioned above or its fullydeflected state where it produces a black optical state.

Thus these binary operated modulators are capable of displaying only twogray levels per pixel, such as black and white in the case of amonochrome modulator, or eight colors per pixel, such as red, green,blue, cyan, yellow, magenta, black, and white for example, in the caseof a color modulator. It is desirable to display additional shades ofgray in a monochrome display and additional colors in the case of acolor display. Since controlling analog deflection of the singlemonochrome mirror per pixel or three-color mirrors per pixel can beunreasonably difficult it becomes necessary to devise a modulatorarchitecture with a more complex pixel structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention may be best understood by reading thedisclosure with reference to the drawings, wherein:

FIG. 1 shows an example of an interferometric modulator.

FIG. 2 shows a prior art implementation of an area-ruled LCD modulatorpixel, including its corresponding connection leads.

FIG. 3 shows an embodiment of an interferometric modulator using areaarray modulation having reduced leads.

FIG. 4 shows a timing diagram for a multiplexed interferometricmodulator.

FIG. 5 shows another embodiment of an interferometric modulator usingarea array modulation having reduced leads.

FIG. 6 shows a timing diagram for an interferometric modulator usingequally weighted regions.

FIG. 7 shows another embodiment of an interferometric modulator usingarea array modulation.

FIGS. 8 a-8 c show an embodiment of electrically cascadedinterferometric modulator elements.

FIG. 9 shows an embodiment of deflectable elements similar tointerferometric modulator elements used as switches.

FIG. 10 shows a graph illustrating how deflectable elements can beselectively addressed by varying amplitude and duration of voltagepulses.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows an example of an interferometric light modulator. Thisparticular example is an iMoD™, but any interferometric modulator mayutilize the embodiments of the invention. No limitation or restrictionto iMoD™ modulators is implied or intended.

The modulator is typically comprised of an array of individual elementsarranged in rows and column. One element shown in FIG. 1 has anelectrode layer 12 on a transparent substrate 10, typically glass. Onesurface 14 of the modulator's optically resonant, interferometric cavityis fabricated on the electrode layer and an oxide layer 16 covers thissurface. The parallel surface of the cavity, mirror 20, is suspendedabove the cavity by supports 18. In operation, when the electrode on theglass substrate is activated, the mirror 20 is electrostaticallyattracted toward the glass substrate. The deflection of the mirror 20changes the dimensions of the cavity and causes the light within thecavity to be modulated by interference.

The resulting picture element (pixel) from a direct-view display will becomposed of elements such as the one shown in FIG. 1. Each of thesemodulator elements with the mirror 20 in an undeflected state will bebright, or ‘ON.’ When the mirror 20 moves to its full design depth intothe cavity toward the front surface of the cavity, the change in thecavity causes the resulting pixel to be ‘dark’ or OFF. For color pixels,the ON state of the individual modulating elements may be white, red,green, blue, or other colors depending upon the modulator configurationand the display color scheme. Most typically a single color pixel willbe composed of a number of modulator elements that createinterferometric blue light, a similar number of elements that createinterferometric red light, and a similar number that createinterferometric green light. By moving the mirrors according to displayinformation, the modulator can produce full color images.

The most basic display activates simultaneously all modulator elementsof a given color within a pixel with the result being that eight colorsper pixel are possible. The current invention provides for theactivation within a pixel of some elements of a given color separatelyfrom other elements of that same color. This enables multipleintensities of red light, multiple intensities of blue light, andmultiple intensities of green light to be mixed within a given pixel.The result is that rather than being limited to 8 colors per pixel theinterferometric display is capable of thousands of colors per pixel.

Similar types of area-weighted modulation have been practiced in othertypes of displays. For example FIG. 2, which corresponds to FIG. 9 inU.S. Pat. No. 5,499,037, shows an area-weighted method. In this example,16 levels of intensity are provided by creating a subpixel containing 9separate pixel elements addressed with six electrodes, three formed ashorizontal connecting leads and three formed as vertical connectingleads. A full-color pixel formed this way might have 9 vertical leads,three for red, three for green, and three for blue, and the same threehorizontal leads shown in FIG. 2. This pixel provides 4096 colors(16×16×16), but with 12 leads it would result in a much more complexdisplay system than would a pixel with four leads providing eightcolors.

Because the individual elements of interferometric modulators tend tooperate in a binary mode, bright in a quiescent state and dark in afully deflected state, analog operation is not readily available.Therefore, interferometric modulators are likely to benefit from anarea-ruled method of operation. It is one purpose of embodiments of thisinvention to provide a method of area-ruled operation that is uniquelysuited for application to interferometric modulators and which reducesthe complexity required by previous implementations.

One embodiment of an interferometric modulator area-ruled architecturerequiring fewer pin outs and still providing higher bit depth is shownin FIG. 3. Driver device 50 has one output pin per display row, and aconnection line is provided between each driver device output pin and acorresponding row of the modulator array. The single row connection ismultiplexed between the sub-elements that comprise the sub-rows of thedisplay element. The term display element has been introduced here tospecify a certain area of the entire display surface. The displayelement is a collection of sub-elements, that typically resolves into aportion of the display presenting a coherent set of image information.The most typical display element would correspond to a single pixel inthe resulting image. The display element 40 in FIG. 3 has been dividedup into three columns, 42, 44, and 46, typically one for each color suchas red, green and blue, in the case where 40 represents a pixel. Inaddition, each column has been divided up into 4 sub-elements arrangedin sub-rows.

To understand how the system in FIG. 3 functions, consider that the rowselect outputs of driver device 50 typically become active in asequential pattern starting with output 1, then proceeding to output 2and so on. When the timing signals cause row 5 to become active, theswitch 56 closes and switch 58 becomes open, causing the active driverpin voltage to be applied to the sub-row of sub-elements 42 a, 44 a and46 a. Simultaneously, the data lines 43, 45, and 47, which may beconnected to a driver device not shown in FIG. 3, are driven with theproper voltages to cause the sub-elements 42 a, 44 a, and 46 a to switchto states appropriate to the current image content associated withdisplay element 40.

As soon as sub-elements 42 a, 42 b, and 42 c have reached their newlyaddressed positions switch 56 opens and switch 58 closes. Immediatelythereafter switch 57 closes and switch 59 opens, and the data lines 43,45, and 47 are driven to the appropriate values for sub-pixels 42 b, 44b, and 46 b. This sequence continues until the three data lines havebeen driven with the four different data sets to update the twelvesub-pixels in display element 40. Then this sequence repeats for rows 6,7, and so on.

As shown in FIG. 3 the timing/switch-activation signals are shared withall other rows throughout the display, so that switches for the firstsub-row of every row toggle when the first sub-row of any row is active,and so on for the second, third, and fourth sub-rows. However, only thedriver output pin for the active row is energized with the activeaddressing voltage level. All non-active rows are held to a non-selectbias voltage while the active row is elevated to a data select voltage.In this manner, elements in all sub-rows except the active sub-row seethe same, non-select voltage independent of the status of the timingsignals and consequently independent of the positions of the switcheswithin the sub-row. It should be observed that switching of thenon-active, row-select switches could be avoided with masking circuitsin cases where ultimate low power consumption is desired.

As can be seen from FIG. 3, the switches 56 and 58 as well as the otherswitches connected to the timing signal lines, are manufactured to bemicroelectromechanical devices similar to the interferometric elements,such as the one shown in FIG. 1. Because the array is undergoingmicroelectromechanical processing to create the interferometric displayelements, manufacturing these ‘extra’ elements in the area surroundingthe array would not create extra complexity or necessarily raise thecost of the device. It is possible that the multiplexing of the sub-rowscould be done with other types of switches, including but not limited tomicroelectromechanical switches fabricated in a manner not similar tothe fabrication of the interferometric elements and more conventionalelectronic switches fabricated using thin silicon films deposited on themodulator's glass substrate.

The term ‘similar to’ as used here means that the devices have the samebasic structure of an electrode, a cavity, and a mirror suspended overthe cavity. When constructing an electrical switch, the opticalfunctionality fabricated near the glass substrate in an interferometricmodulating element is not required, and it may be desirable to eliminatethis optical functionality. It is only required that at full deflectionthe mirror come in contact with (and hence connect electrically) twoconductive areas, most likely fabricated from the thin film layers usedto fabricate the addressing electrodes and/or a conductive layer used toform the front wall of the optically resonant cavity. This is differentthan the way the interferometric element may operate, which is why theswitch structure is ‘similar to’ rather than the same as the displayelement.

A timing diagram for one possible operation of row 5 is shown in FIG. 4.At t₀, the signals for row 4 are high, as seen from the modulator. Attl, the signal for row 4 goes low and row 5 becomes active. Similarly,the line for row 5A becomes active. At t₂, row 5A goes low and row 5Bbecomes active. This continues in series for rows 5C and 5D.

This embodiment results in a reduced number of connections between thearray and the driver. The connections necessary to drive the individualsub-elements in the sub-rows are made in the surrounding area of themodulator array, bringing them ‘on-chip’ with the array, rather thanrequiring the driver device to provide separate output pins for eachsub-element.

The sub-elements shown in FIG. 3 have been drawn to be of approximatelyequal areas. Another useful geometry uses sub-elements with binary‘physical’ weighting relative to each other. As can be seen in thedisplay element 70 of FIG. 7, a bit depth of 4 is provided using only 4such sub-elements. For example, area 78 has a size approximately equalto one-half the full size of display element 70. Sub-element 74 has asize approximately equal to one-half the size of the next largestsub-element, in this case, sub-element 78, giving sub-element 74 a sizeone-quarter of the full-size display element. Each subsequentsub-element has a size approximately equal to one-half the size of thenext largest sub-element. Sub-element 72 is one-half that of 74, orone-eight that of 70. Sub-element 71 is one half of 72, or one-sixteenthof 70.

The elements having different physical sizes are activated as needed toachieve an overall display pixel with a given color intensity. The tablebelow shows the ON sub-elements, by reference number for each colorintensity level. Color intensity Sub-elements ON 0 None 1 71 2 72 3 71,72 4 74 5 71, 74 6 72, 74 7 71, 72, 74 8 78 9 71, 78 10 72, 78 11 71,72, 78 12 74, 78 13 71, 74, 78 14 72, 74, 78 15 71, 72, 74, 78

Although described here as being addressed by the multiplexing techniqueof FIG. 3 this implementation of area weighting can be used separatelyfrom the multiplexing technique described above. Increasedinterconnection complexity will result, but this relatively low level ofcomplexity will be acceptable in many systems.

In an alternative embodiment, the bit depth of 4 is achieved by dividingeach sub-column for a display element into 16 (2⁴) sub-elements. Eachset of 16 sub-elements in each sub-column are connected together in acascading fashion and are therefore referred to as ‘sub-elementcascades.’ The individual cascaded sub-elements maybe fabricated toserve as both interferometric modulating elements and electricalswitches such as those shown in FIG. 3. Alternatively, each individualinterferometric sub-element may have an electrical switch fabricatedimmediately adjacent.

An example of a display element 60 having sub-element cascades for 4bits of depth is shown in FIG. 5. The three column lines connect to thefirst element in a sub-element cascade; sub-element 61 r is the firstelement in the red cascade, 61 g in the green cascade and 61 b in theblue cascade, the last elements being 615 r and 616 r for the redcascade. The control of color intensity is provided by the width of anaddressing pulse applied to the column line. This may be best understoodby looking at two possible sub-element configurations shown in FIG. 8and FIG. 9.

In FIG. 8 a sub-element members of a sub-column, such as thesub-elements 61 r, 62 r, and 63 r shown in FIG. 5, are shown incross-section. The interferometric cavity is defined by the suspended,movable mirror elements such as 82 and the front surface optical filmstacks such as 84. In this case the suspended movable element alsofunctions as the contactor of a switch as did the switches shown in FIG.3. Arrayed within each sub-element, adjacent to the optical film stack,are conductive elements 86 a-86 e. When the movable mirror 82 comes incontact with the optical film stack 84 the sub-element has switched fromone optical state to a second optical state. Additionally, a circuit iscompleted, since the movable mirror now connects conductor 86 a toconductor 86 b.

The sub-element cascade is addressed by applying one polarity of anaddress voltage to the moving mirror 84 and the fixed contact 86 a. Thesecond polarity of the address voltage is applied to an electrode withinor below the optical film stack 84. The resultant potential differencecauses mirror 82 to deflect, completing the connection between conductor86 a and 86 b as shown in FIG. 8 b. With the first voltage polarity nowapplied to conductor 86 b (through mirror 82), mirror 92 will eventuallydeflect as shown in FIG. 8 c. This process will continue until all ofthe cascaded mirrors have collapsed or until the address pulse isremoved. Thus the reflective intensity of the display element iscontrolled by controlling the time duration (or pulse width) of theaddress pulse.

FIG. 6 shows a timing diagram for three successive addressing sequencesof a sub-element cascade with color values of 12, 13, and 3respectively. A color value of 0 is ‘black’ or OFF, and a color value of16 would represent all sub-elements of the cascade being on. Theaddressing pulse is shown in the top line. As can be seen, as the timeduration of the address pulse increases more mirror elements areactivated, moving into the ‘black’ or OFF state. During the firstaddress pulse, four elements move into the ‘black’ state, and a colorvalue of 12 is achieved—12 elements are left in the bright state. Takenin light of FIG. 5 this might correspond to sub-elements 61 r, 62 r, 63r, and 64 r switching successively to the ‘black’ state.

During the second addressing, the address pulse is shorter, and onlythree elements switch to the ‘black’ state. In the final addressing, 13elements switch to the ‘black’ state leaving a fairly dark reflectancecolor value of three. The discontinuity in the timing diagram representsthe relatively long period (typically a ‘frame time’ in video terms)that the mirrors remain in their addressed states. It is during thisintegration time that the viewer's eye becomes impressed with the areaweighted intensity value.

At the end of each frame time, FIG. 6 shows that all of the mirrors arereset to their quiescent position before being addressed again. It ispossible to address the interferometric device so that this reset is notnecessary. It is included here to emphasize the switching operationsthat take place during each ‘line time’ of addressing. It should benoted that the address pulse must exceed a certain minimum time durationin order to cause the first element in a sub-element cascade to turn ON.A very short addressing pulse, such as a transient signal, that isshorter than the response time of the sub-elements, will not cause thefirst sub-element to switch.

Once the address pulse has been active long enough for the firstsub-element to switch, the addressing signal is ‘passed on’ to the nextelement in the array. Again, the address pulse must be active longenough past the switching of the first element to cause the secondelement to switch. As the response times of the sub-elements are assumedto be approximately the same, the sub-element cascade should becontrollable to provide a desired number of sub-elements to turn ONwhile providing a relatively high immunity to spurious signals. Thecumulative effect is to cause the display element to form the propercolor intensity in the resultant pixel. In this manner, a displayelement with a bit density of 4 was made possible without any extraconnection lines or extra connections from the driver device.

The cascade effect described above is based upon movable mirror elementsthat provide both the optical function of the display as well as theelectrical switching cascade itself. An alternative, shown in FIG. 9, isto provide adjacent to each interferometric sub-element such as 100 aseparate electrical switch such as 102 that toggles simultaneously withthe optical element. The embodiment of FIG. 9 shows a micromechanicalswitch, but other types of switches, such as silicon or othersemiconductor transistor switches may be used as well. In this way theparameters of the optical element and the parameters of the electricalelement can be separately optimized. The addressing waveforms and colorvalue results of the system of FIG. 9 are identical to those provided bythe system of FIG. 8. The systems of FIGS. 8 and 9 are both examples ofobtaining different levels of bit-depth by controlling the behavior ofthe modulator over time.

In FIG. 10 a graph is provided to illustrate that by varying the timeduration of addressing pulses as well as the voltage levels of thosepulses a more refined control of movable mirror addressing for bit-depthswitching can be implemented, FIG. 10 applies to a display elementconsisting of several individual moving mirrors. The mirrors are denotedby the names b₁, b₂, b₃, etc. The mechanical support structure of themirrors as well as the mirror element themselves can be manufacturedusing a number of different techniques, such as varying film thicknessand residual stress within the film, to allow individual mirrors todeflect at different rates versus time and at different displacementsversus applied voltage,

As shown in FIG. 10 movable element b₁ deflects after a shortapplication of voltage V₁. Mirror b₂ responds more slowly and willactivate after a longer application of V₁. Both mirrors b1 and b3 willrespond to a brief application of V₂ and mirror b₄ is capable ofresponding to a very quick application of V₂ to which none of the othermirrors can respond. In this way various combinations of mirrors can bedeflected by shaping address pulses in the time/voltage space, where theterm ‘combination’ includes the switching of a single element. If thesemirrors have different areas, such as the areas shown in FIG. 7, thenmultiple brightness levels can be achieved by addressing a multi-segmentdisplay element with a single pair of electrical connections.

In all of these manners, alternative methods of providing intensity bitdepths far beyond a single bit can be accomplished for interferometricelements. While the implementations above were discussed with regard toa bit-depth of 4, having 16 levels of color intensity, these embodimentsmay be employed for any bit-depth greater than 1.

Thus, although there has been described to this point particularembodiments for a method and apparatus for area array modulation andreduced lead count in interferometric modulators, it is not intendedthat such specific references be considered as limitations upon thescope of this invention except in-so-far as set forth in the followingclaims.

1. A method of operating a MEMS device, comprising: receiving a firstsignal for a plurality of elements; transmitting the first signal for afirst predetermined period of time to some of the elements such that afirst predetermined number of the elements are activated.
 2. The methodof claim 1, further comprising transmitting a second signal for a secondpredetermined period of time to some of the elements such that a secondpredetermined number of the elements are activated.
 3. The method ofclaim 2, wherein the second predetermined period of time is greater thanthe first predetermined period of time and the second predeterminednumber is greater than the first predetermined number.
 4. A MEMS device,comprising: a plurality of elements having differing values of at leastone of activation versus time and activation versus voltage; andaddressing lines configured to provide signals of at least one ofselectable voltage and selectable time to the elements such thatdifferent numbers of elements activate in a selectable manner, accordingto the voltage and time of the signals.
 5. The device of claim 4,wherein the addressing lines are configured to provide one signal for arow in the plurality of elements.
 6. The device of claim 4, wherein theelements are bi-stable.
 7. The device of claim 4, wherein the elementsare light modulators.
 8. The device of claim 4, wherein the elementscomprise differing mechanical structures so as to effect differingvalues of at least one of activation versus time and activation versusvoltage.
 9. The device of claim 8, wherein the elements comprise atleast one of differing mechanical support structure, differing filmthicknesses, and residual stress.
 10. The device of claim 4, wherein theelements have differing areas.
 11. A MEMS device, comprising: aplurality of elements having differing values of at least one ofactivation versus time and activation versus voltage; and means forproviding signals of at least one of varying voltage and varying time tothe elements such that different numbers of elements activate in aselectable manner, according to the voltage and time of the signals. 12.The device of claim 11, wherein the providing means comprises addressinglines.
 13. The device of claim 11, wherein the elements are lightmodulators.
 14. The device of claim 11, wherein the elements comprisediffering mechanical structures so as to effect differing values of atleast one of activation versus time and activation versus voltage. 15.The device of claim 11, wherein the elements comprise at least one ofdiffering mechanical support structure, differing film thicknesses,differing area and differing residual stress.
 16. A method ofmanufacturing a MEMS device, the method comprising: forming a pluralityof elements having differing values of at least one of activation versustime and activation versus voltage; and connecting addressing lines tothe plurality of elements, wherein the addressing lines are configuredto provide signals of at least one of selectable voltage and selectabletime to the elements such that different numbers of elements activate ina selectable manner, according to the voltage and time of the signals.17. The method of claim 16, wherein the addressing lines are configuredto provide one signal for a row in the plurality of elements.
 18. Themethod of claim 16, wherein the elements are bi-stable.
 19. The methodof claim 16, wherein the elements are light modulators.
 20. The methodof claim 16, further comprising forming elements of differing mechanicalstructures so as to effect differing values of at least one ofactivation versus time and activation versus voltage.
 21. The method ofclaim 20, further comprising forming elements comprising at least one ofdiffering mechanical support structure, differing film thicknesses,differing area and differing residual stress.